The Yarn Weight Chart below gives a handy comparison of different knitting and crochet yarns along with needle and hook sizes, approximate meterage to the 100g and WPI, using South Africa, UK, US, German and Australian terminology.

Yarn weight is important in achieving the correct gauge or tension for a particular project and can help with yarn substitution. Most yarns state their weight on the ball band but some may not,this is particularly the case with Bulgarian and Turkish yarn manufacturers.

However by looking at the meterage,and recommended needle size in this yarn weight chart, which are also given on the ball band, it is possible to determine the weight of any given yarn.

Another useful method of determining weight is by, wraps per inch, or wpi.  To do this wrap the yarn around a large needle or a ruler. Make sure that the yarn lies flat and push the yarn together so that there are no gaps between the wraps. Smooth it out so it is neither too loose nor too tight then measure the number of wraps per inch (2.5 cm). For better accuracy, measure the wraps at the center of your yarn sample.

The number of wraps will act as a gauge to assess the thickness of unmarked yarn; for example 12 wraps per inch is 12 wpi, and is used to calculate how much yarn is required for various articles,so that 12 wpi is equivalent to DK,8 ply (worsted weight, medium weight) yarn.

Below is a table comparing yarn weights (ply or thicknesses) in a range of countries

USA South Africa / UK Australia Germany m/100g Wraps Per Inch Recommended knitting needle size, mm Recommended crochet hook size, mm Other terms used
0 or Lace 1 ply More than 800 40+ wpi 1.5 - 2.5 1.5 - 2.5 Single, Cobweb, Thread, Zephyr
0 or Lace 2 ply 2 fädig (ply) 600-800 30-40 wpi 1.5 - 2.5 1.5 - 2.5
1 or Super Fine 3 ply 3 ply 3 fädig 500-600 20-30 wpi 2 - 3 2.25 - 3.5 Light Fingering, Sock, Baby
1 or Super Fine 4 ply 4 ply 4 fädig 350-450 14-24 wpi 2 - 3 2.25 - 3.5 Fingering, Sock, Baby
2 or Fine 5 ply 6 fädig 250-350 12-18 wpi 3 - 4 3.5 - 4.5 Sport, Baby, 3-ply (obsolete American)
3 or Light DK (Double Knit) or 8 ply 8 ply 200-250 11-15 wpi 4 - 4.5 4.5 - 5.5 Light Worsted, DK
4 or Medium Worsted, Aran, Triple Knit (rare) 10 or 12 ply 120-200 9-12 wpi 4.5 - 5.5 5.5 - 6.5 Worsted, Afghan, Fisherman, 4-ply (obsolete American)
5 or Bulky Chunky, Double Double Knit (rare) 12 or 16 ply 100-130 6-8 wpi 5.5 - 8 6.5 - 9 Craft, Rug
6 or Super Bulky Super Chunky, Seriously Chunky Less than 100 5-6 wpi >8 >9 Roving
7 or Jumbo Less than 100 n/a 12.75 mm and larger 15 mm and larger Roving

Types of yarn weights:-

0 - LACE (1 or 2 ply)
Yarns: fingering, thread
Used to make: Doilies or other delicate projects.

1 - SUPER FINE (3 or 4 ply)
Yarns: sock, fingering and baby
Used to make: baby items, socks, airy shawls and wraps.

2 - FINE 
Yarn: sport and baby
Used to make: baby items, lightweight throws.

3 - LIGHT (Double Knit)
Yarns: DK, light worsted
Used to make: baby clothes, socks and other
lightweight clothing

4 - MEDIUM (Aran)
Yarns: worsted, afghan, aran
Used to make: practically anything — this is the
most common yarn weight. If you're a beginner,
it's a great weight to practice your stitches with.

5 - BULKY (Chunky)
Yarn: rug yarn, chunky, craft
Used to make: home decor, bulky winter items
like hats and scarves.

6 - SUPER BULKY (Seriously Chunky)
Yarns: roving
Used to make: heavy scarves, hats, home decor items or anything else you would like to knit up quickly.